1. Field of the Invention
The present invention relates to an electric power steering apparatus. More specifically, the present invention relates to an electric power steering apparatus in which, even if vibration and noise are generated from a motor, between the motor or a pulley, and between a belt and the pulley in the process of transmitting a torque of the motor, the vibration and the noise are reduced and prevented from being transferred to a peripheral component, and during the assembly, even if the motor pulley and a nut pulley are not parallel with each other, the belt is mounted in a distorted state, or the tension of the belt is fluctuated. The teeth of the belt and the teeth of the pulley are stably engaged with each other such that the vibration and the noise are prevented from being generated between the belt and the pulley, and power consumption is reduced, which enables a steering assist force to be uniformly transmitted to a driver.
2. Description of the Prior Art
FIG. 1 is a view schematically illustrating a configuration of a conventional electric power steering apparatus. FIG. 2 is a schematic cross-sectional view of a portion of FIG. 1.
As illustrated in these drawings, a conventional electric power steering apparatus includes: a driving unit 120 controlled by an Electronic Control Unit (ECU) configured to determine a steering extent of a steering wheel 113 through a torque sensor 111 mounted on a steering column 110; a rack bar 130 configured to move a tie rod 133 connected to a wheel 131 according to the steering extent of the steering wheel 113; and a driven unit 140 configured to receive a rotational force transmitted from the driving unit 120, to convert the rotational force into an axial moving force, and to transmit the axial moving force to the rack bar 130.
Here, as illustrated in FIG. 2, the driving unit 120 includes an electric motor 210 controlled by the ECU, a driving pulley 211 fixed to the motor shaft 212 of the electric motor 210, and a belt 213 wound on the driving pulley 211.
In addition, the driven unit 140 includes a ball nut 230 configured to support the rack bar 130 within the rack housing 220 that encloses the rack bar 130, and a driven pulley 240 and the nut pulley 250 are assembled to the outer peripheral surface of the ball nut 230.
Inner peripheral threads having a hemispherical cross-section are formed in a spiral shape on the inner peripheral surface of the ball nut 230, outer peripheral threads having a hemispherical cross-section are also formed in a spiral shape on the outer peripheral surface of the rack bar 130, and a ball is inserted between the inner peripheral threads and the outer peripheral threads such that the rack bar 130 is moved left and right when the ball nut 230 is rotated.
In the electric power steering apparatus, the driving pulley 211 connected to the motor shaft 212 of the electric motor 210 and the driven pulley 240 coupled to the ball nut 230 are arranged to be parallel with each other, and the belt 213 are wound on the driving pulley 211 and the driven pulley 240 such that the rotational force of the electric motor 210 is transmitted to the rack bar 130, and the rack bar 130 is moved left and right by the operation of the ball nut 230 to generate an auxiliary force.
However, the conventional electric power steering system configured as described above has a problem in that in the process of transmitting the torque of the motor, vibration and noise are generated from the motor, between the motor and the pulley, and the belt and the pulley, and the vibration and noise are transferred to a peripheral component to make a driver feel tired and unpleasant.
In addition, when the belt is mounted in a distorted state or the teeth of the belt and the teeth of the pulley are engaged to be deviated from each other because the motor pulley and the nut pulley are not parallel with each other during the assembly, there is a problem in that the belt is expanded in a diagonal direction such that one side of the belt in the width direction is disengaged or separated from the teeth of the pulley, thereby generating vibration and noise.
Further, there are problems in that the teeth of the belt and the teeth of the pulley are not stably matched with each other due to the change of the tension of the belt, and thus vibration and noise are generated and power loss is increased in the process of transmitting the torque of the motor. Consequently, a steering assist force is not uniformly transmitted to the driver, which makes the driver feel inconvenienced.